Yell Budgeting Tool: Rollover for new month

As part of the rollover on the first of the month, if the cost target for this
month = 0, the cost target for the previous month should be used. Example:

Example:
Current date = Feb 1. We migrate the cost targets for 2010-01-01 (January) into
the "previous month", and we migrate the cost targets with a date of 2010-02-01
(February) into the "current month" cost target. However, if there are campaigns
that are not deleted, we need to carry over the cost target from January into
February.